As an electrostatic vibration type device is proposed an electrostatic vibration type angular velocity sensor in which a base portion, a vibrator and an exciting electrode for driving the vibrator are formed by etching a semiconductor substrate (for example, see JP-A-2001-91265 (pp4–5, FIG. 1)).
According to this angular velocity sensor, the vibrator is driven (excited) to vibrate in a predetermined driving direction by the driving electrode, and under application of an angular velocity, the vibrator is vibrated in a direction (detecting direction) perpendicular to the driving direction by Coriolis force. The applied angular velocity is detected on the basis of the vibration in the detecting direction.
The present invention has manufactured a prototype of an electrostatic vibration type angular velocity sensor as described above, and made various studies on the prototype.
FIG. 7 is a schematic plan view showing the construction of an angular velocity sensor as a first prototype manufactured on the basis of the related art described above.
This prototype may be manufactured by a well-known semiconductor manufacturing technique using a SOI (silicon on insulator) substrate 10 having two silicon substrates attached to each other through an oxide film. FIG. 7 also shows one silicon substrate (semiconductor substrate) 12 having a planar shape. Grooves are formed on the one silicon substrate 12 by etching to form respective parts.
A vibrator 30 is disposed on an opening portion 14 which is formed by partially removing the oxide film supporting one silicon substrate 12 and the other silicon substrate. The vibrator 30 is fixed to a base portion 20 located around the outer periphery of the opening portion 14 through driving beams 33 which are designed to be deformable elastically (like spring) in an x-direction of FIG. 7. The base portion 20 is formed of the oxide film and the other silicon substrate.
Furthermore, comb-shaped driving electrodes 40, 41 for applying electrostatic force to the vibrator 30 to drive and vibrate the vibrator 30 in the x-direction are fixed to the base portion 20. The driving electrodes 40, 41 are disposed so as to confront the outer peripheral portion of the vibrator 30. In addition, comb-shaped portions 30a are equipped to the vibrator 30 at the portions thereof facing the driving electrodes 40, 41 so that the comb-shaped portions 30a are engaged with driving electrodes 40, 41.
In FIG. 7, a detecting poise portion 32 is formed at the center portion of the vibrator 30 so as to be linked to the right and left sites (plane-shaped portions) of the vibrator 30 through detecting beams 34 deformable in the y-direction (like spring). Furthermore, detecting electrodes 50 are formed at the base portion 20 so as to face the detecting poise portion 32.
In the electrostatic vibration type angular velocity sensor shown in FIG. 7, when a constant voltage is applied to the vibrator 30 and alternating voltages (driving signals) which are opposite in phase are applied to the right and left driving electrodes 40, 41 respectively, the overall vibrator 30 is driven to vibrate in the x-direction by the driving beams 33.
When an angular velocity Ω is applied under the driving-vibration of the vibrator 30, Coriolis force acting in they-direction occurs in the vibrator 30, and the detecting poise portion 32 supported by the detecting beams 34 in the vibrator 30 vibrates in the y-direction by the Coriolis force (hereinafter referred to as “detecting-vibration”). At this time, the electrostatic capacitance between the detecting electrodes 50 and the detecting poise portion 32 is varied due to the detecting vibration. The magnitude of the angular velocity Ω can be determined by detecting the capacitance variation.
Since Coriolis force is proportional to the vibration speed of the driving-vibration of the vibrator 30, it is required to increase the vibration speed in order to enhance the sensitivity of the angular velocity and thus detect the angular velocity with high precision. Therefore, it is required to increase the number of driving electrodes and thus intensify the driving force, that is, the electrostatic force. For example, it is required in the sensor shown in FIG. 7 to increase the number of comb teeth of the driving electrodes 40, 41.
However, if the number of driving electrodes is merely increased, the body size of the substrate constituting the sensor is increased, and this is unfavorable. Therefore, the inventor of this invention has produced an electrostatic vibration type angular velocity sensor as shown in FIG. 8 as a second prototype.
The second prototype is achieved by modifying the right and left sites of the vibrator 30 of the first prototype shown in FIG. 7 so that each plane-shaped portion is designed as a frame portion 31 having a frame shape. Furthermore, a part of each of driving electrodes 40, 41 fixed to the base portion 20 is located in the inner space surrounded by the inner periphery of each frame portion 31 and serves as an in-frame fixed portion 60, so that each of comb-shaped driving electrode 40b, 41b is also equipped to the in-frame fixed portion 60 surrounded by the frame portion 31.
That is, the driving electrodes 40, 41 of the second prototype comprise the first driving electrodes 40a, 41a which are disposed so as to confront the outer peripheral portion of the vibrator 30, and the second driving electrodes 40b, 41b which are equipped to the in-frame fixed portions 60 so as to confront the inner peripheral portions of the right and left frame portions 31, respectively.
As described above, the number of driving electrodes which are designed to have a high area efficiency can be increased by adopting the structure that the vibrator 30 is designed in a frame-shape structure and the driving electrodes are also equipped in the inner spaces surrounded by the inner peripheral portions of the frame portions 31. Accordingly, the electrostatic force applied to the vibrator 30 may also be increased by the increased number of the comb teeth because of provision of the second driving electrodes 40b, 41b. 
In the construction of FIG. 8, however, electrostatic force acting in the opposite direction to the electrostatic force caused by the driving-vibration acts in the gap (hereinafter referred to as “confronting back-side gap”) 70 between the back side portion 60a of each in-frame fixed portion 60 at the opposite side to the teeth-arrangement portion of the corresponding second driving electrode 40b, 41b and the inner peripheral portion of the corresponding frame portion 31 facing the back side portion 60a. 
Accordingly, even when the number of driving electrodes is increased, the driving force and the driving-vibration speed are not increased in conformity with the increase of the number of driving electrodes. This problem occurs commonly not only when increase of the driving force is intended in the electrostatic vibration type angular sensor, but also when it is intended in the electrostatic vibration type device having the structure that the vibrator is designed in a frame shape and the driving electrodes are equipped in the frame.